Dj. Collins et Ec. Turner, AN AUTOMORPHISM OF A FREE GROUP OF FINITE RANK WITH MAXIMAL RANK FIXED-POINT SUBGROUP FIXES A PRIMITIVE ELEMENT, Journal of pure and applied algebra, 88(1-3), 1993, pp. 43-49
We prove that if an automorphism phi of a free group F of rank n has a
fixed point subgroup of rank n then phi fixes a primitive element of
F. With an appropriate generalisation of rank and primitivity, we prov
e the same statement for automorphisms of free products.